In a conventional textile process, the yarn may be fed to a textile machine, e.g., a circular knitting machine, by a plurality of so-called “storage” yarn feeders. A storage yarn feeder is generally provided with a drum having a plurality of yarn loops wound thereon, which are adapted to be unwound upon request from the downstream machine As the yarn is unwound from the drum, it may be re-loaded either by a motorized swivel arm rotating about an axis coaxial with the axis of the drum, or, in the case of feeders considered here, by driving the drum to rotate, which drum, in this case, must be motorized.
During the feeding process, it is very important to maintain the amount of yarn stored on the drum substantially constant on an optimum level, as well as to maintain the loops regularly spaced from each other. In fact, a reduction of the stock below an optimum level would cause the yarn tension to rise excessively, resulting in defects in the finished product. In extreme cases of a stock reduced to zero, the downstream machine would start drawing yarn directly from the reel, which circumstance would cause unacceptable peaks of tension. On the contrary, a growth of the stock above an optimal level would cause the yarn to accumulate at the delivery end of the drum, with the yarn loops overlapping unevenly and consequent anomalies in the feeding process.
Such phenomena are very difficult to be controlled, particularly when the feeders are installed on large-in-diameter circular knitting machines, which may have even more than sixty feeders installed thereon.
In order to limit the above drawbacks, it is known to control the amount of stock on the drum.
A simple control method consists of providing the feeder with sensor means, e.g., optical sensors or mechanical sensors, which are adapted to provide a binary information about the presence/absence of yarn at a predetermined area of the drum. The rotation of the drum is controlled on the basis of the signals generated by the above sensor means in such a way as to maintain the stock on the drum within the monitored area.
The above control system, which is based on a binary information about the presence/absence of yarn in a monitored area of the drum, allows the tension of the yarn delivered by the drum to be controlled only approximatively, because the stock oscillates continuously within a predetermined range with a relatively high amplitude. This circumstance inevitably affects the regularity of the yarn-feeding process and, consequently, the quality of the finished tissue.
More sophisticated control systems are also known, in which the amount of stock on the drum is estimated on the basis of an information about the number of loops which are unwound from the drum and an information about the number of loops which are wound on it, both such items of information being provided by sensor means, e.g., optical sensors, from which relative (i.e., non-absolute) items of information can be derived. A system of this type is described, e.g., in EP 2 592 032. In this case, the rotation of the motor is controlled in such a way as to maintain the amount of yarn substantially constant with respect to a predetermined amount of yarn which is wound on the drum during an initial loading procedure, which is also described in the above document.
Theoretically, the above system allows the amount of yarn stored on the drum to be controlled very accurately. However, as it is based on the comparison between two relative items of information, in the practice it has the drawback that it is vulnerable to detection errors of the sensors (which errors may be caused, e.g., by signal noise or dust in the environment). In the presence of such errors, a so-called “drift” phenomenon may occur, which is well known to the person skilled in the art, in which the stock tends to rise or diminish in an uncontrolled way (i.e., without the system noticing it and intervening by compensating the error), up to a complete emptying or overloading of the drum.
The above vulnerability is also evident in the case of a temporary interruption of the power. In fact, after the interruption, the drum will continue to rotate by inertia, thereby winding a few loops upon itself; however, this information does not reach the control system because the sensors are not powered. Therefore, as the power is restored, the control unit will start modulating without compensating this accidental increase in the stock.